If you read the README of the Google Docs module it states: "This extension adds the ability to edit supported content items in Google Docs™ to the Alfresco repository and Share interface." So this allows you to export a document from Alfresco into Google Docs (Google Drive) for editing (potentially simultaneously with other users) and pull those changes back into Alfresco. It does not mention importing pre-existing documents from Google Drive into Alfresco. This is exactly the restriction that Jeff Potts highlighted in his response to the thread you linked.